When a metallic member is magnetized, a leakage flux is generated at any point where a discontinuity, flaw, or defect in the material exists. The quantity of leakage flux, and therefore the sensitivity of sensing devices to the defect is dependent upon the relative orientation of the defect and the field. If narrow cracks are parallel to the magnetic field, virtually no flux leakage is generated and the defect may not be detected.
The detection of all defects regardless of orientation has generally required that the object under test be inspected more than once, as is taught in U.S. Pat. No. 3,287,632 to Tompkins. The apparatus of Tompkins applies magnetic flux in two directions relative to a tubular member separate and apart from one another to avoid interference and inaccurate data resulting from mingling and vector addition of magnetic field (see column 1, lines 66-70).
Complex apparatus for detecting defects at various orientations have been developed. U.S. Pat. No. 3,994,911 to Tornblom discloses an apparatus in which a multiphase generator is used to generate a traveling magnetic field such as a rotating or linearly traveling field in the body being tested and in which means are provided to sense any assymetry in the magnetic field which results from eddy currents induced in the object. A Y-connected multiphase generator is used to feed a first plurality of Y-connected transducer coils comprising at least one coil per phase. A second plurality of Y-connected transducer coils connected to the multiphase generator induces a moving field in the body in a direction opposite to that of the moving field generated by the first coils. Still more coils are required to detect variations in the electric field generated when faults and irregularities in the body being tested are encountered.
A similar apparatus is disclosed in U.S. Pat. No. 3,855,530 to Fuji et al. A metal defect detecting apparatus is disclosed comprising a cross-shaped magnetic core assembly prepared from two pairs of magnetic pole members which provide two pairs of magnetic poles intersecting each other at right angles, a first and second coil wound about the two pairs of magnetic pole members, and a power supply for supplying the first coil with single phase alternating current of positive half wave and the second with single phase alternating current of negative half wave, thus alternately supplying each of the two magnetic fluxes in perpendicularly intersecting directions. A leakage magnetic flux representing a vector sum of the magnetic fluxes occurs when a defect obliquely intersects either of the fluxes (see column 3, line 65 to column 4, line 19). This apparatus is intended to be used with magnetized powders.
Various arrangements using perpendicular magnetic fields have been exploited in the field of eddy current testing. For example, U.S. Pat. No. 3,495,166 discloses eddy current crack detection systems using crossed coils with fields perpendicular to each other and parallel to the surface of a part. Variations in conductivity, permeability, lift-off, and surface conditions produce substantially no output from the bridge detector circuit network, but defects produce a large output.
Another eddy current detection apparatus is disclosed in U.S. Pat. No. 4,134,062 to Woodbury. This device is designed primarily for rapid scanning of the interior walls of tubes or holes in metal parts in order to detect faults such as fissures, cracks, or other defects. It incorporates sensing transformer elements in the probe differentially connected so as to cancel out and reject from the sensing circuit the effects of false responses due to transitional positioning of the probe, such as those that can occur when the probe is being inserted and removed from a test probe (see column 1, lines 43 to 50).
While the detection of these defects having various orientations within the object under test may be accomplished using these and possibly other prior art systems, outputs are generally not developed which are indicative of the nature, orientation and size of the defect unless more than one test operation is performed. In addition it is difficult to distinguish between signals generated by defects and those generated by normal variations in the contour of the part such as changes in diameter at shoulders.